1. Field of the Invention
The present invention relates to a control method in an induction motor and more particularly to a field weakening control method in induction motor for controlling reverse electromotive force of an induction motor.
2. Description of the Prior Art
Generally, induction motors generate reverse electromotive force when they are operated according to rotating speed.
When the reverse electromotive force is increased in proportion to a rotary speed, there sometimes occurs a case where the reverse electromotive force becomes greater than driving voltage applied to the motor, such that in order to avoid this kind of occurrence, a field weakening control is enforced to decrease the flux in reverse proportion to rotating speed of the motor. In other words, decrease of the flux in reverse proportion to the rotating speed of the motor serves to decrease torque and to prevent the reverse electromotive force from growing larger than the driving voltage.
However, when the flux is decreased in reverse proportion to rotating speed, there occurs too much loss of driving torque at a high speed region, such that field weakening region is controlled as below in order to maximize the driving torque at the high speed region. Rotor flux of an induction motor in the control of field weakening region is shown as per the following formula 1. EQU .lambda..sub.dr.sup.e =L.sub.m i.sub.ds.sup.e [Formula 1]
where, PA1 where, PA1 where,
.lambda..sub.dr.sup.e : rotor flux PA2 L.sub.m : magnetization inductance and PA2 i.sub.ds.sup.e : current for synchronized coordinate flux PA2 i.sub.ds.sup.e : current for synchronous coordinate flux PA2 i.sub.qs.sup.e : current for synchronous coordinate torque PA2 V.sub.ds.sup.e : voltage for synchronous coordinate flux, and PA2 V.sub.qs.sup.e : voltage for synchronous coordinate torque. PA2 r.sub.s : stator resistance value PA2 .omega..sub.e : revolution PA2 L.sub.s : stator inductance, and PA2 L.sub..sigma. : leakage inductance
The rotor flux according to Formula 1 can be obtained by simultaneous solutions according to voltage and current limiting condition of invertor determined by Formula 2 and stator simultaneous coordinate voltage equation according to Formula 3. EQU V.sub.ds.sup.e2 +V.sub.qs.sup.e2 .ltoreq.V.sub.s.sup.2.sub.max EQU i.sub.ds.sup.e2 +i.sub.qs.sup.e2 .ltoreq.I.sub.s.sup.2.sub.max,[Formula 2]
V.sub.s max which is a voltage limiting value in the above formula 2 is defined by general inverter direct current voltage and voltage modulating method, and I.sub.s max which is a current limiting value is obtained by semiconductor element thermal rating and current rating. Here, if a rotor flux of an induction motor is operated at a normal condition, an equation for obtaining an induction motor rotor synchronous coordinate voltage can be given by the following formula 3. EQU V.sub.qs.sup.e =r.sub.s i.sub.ds.sup.e +.omega..sub.e L.sub.s i.sub.ds.sup. e EQU V.sub.ds.sup.e =r.sub.s i.sub.ds.sup.e -.omega..sub.e L.sub..sigma. i.sub.qs.sup.e [Formula 3]
As mentioned above, when the Formulas 2 and 3 are united, a voltage limit at current plane is expressed in an oval and a current limit is defined in a circle, as illustrated in FIG. 3, where, an inner joint region of the oval and the circle represents a current command corresponding to an operational condition.
Of course, the above voltage equation of stator synchronous coordinate is given under an resumption that rotor flux is in normal state (namely, speed and load are under a constant state). Rotor flux and current for torque portion at a field weakening region No. 1 where a juncture between the oval and the circle is formed are expressed by Formula 4 and rotor flux and current for torque portion at a field weakening region No. 2 where a juncture between the oval and the circle is not formed (super high speed condition) are defined by Formula 5. ##EQU1##
In other words, the above weakened field control method for induction motor assumes that a motor is in a normal operational state, where same reverse electromotive force is decreased from the entire motor operational regions according to Formulas 4 and 5, thereby expressing the rotor flux and current for torque portion.
Here, speed arrived characteristic experimental value of the induction motor is (1-X) which is the time reaching 4,400 rpm as illustrated in FIG. 4, where torque and flux from predetermined curves.